EMBRYO 31 



parent thallus (Fig. 14): as it , grows, leaf, stem, and root become 

 differentiated (Fig. 15), which finally emerge; at first the embryo Fern- 

 Plant is dependent for its nourishment upon the prothallus which embeds 

 it ; but as the first leaf expands it begins to exercise the assimilating 

 function, which is taken up also by the later formed leaves. The first 

 root also projects into the soil, and soon functionates as an absorbing 

 organ : it is followed later by others. Thus the young plant soon 

 becomes physiologically independent of the prothallus, which rots away, 





FIG. 15. 



Embryo of Adiantutn concinuntn, older than that in Fig. 14. 

 S= stem ; Ffoot. (After Atkinson.) 



leaf; /? = root; 



leaving the young plant established on the soil. It gradually attains the 

 mature characters similar to those of the parent from which it originated. 

 The above is a bare statement of the salient events in the life-cycle, 

 or ontogenetic period of a Fern, as it is seen in its simplest form : and, 

 the adjoining diagram may serve to present them graphically to the eye 

 (Fig. 1 6). The two most notable points are those where the new 

 individual is represented by a single cell, viz. the spore, and the 

 fertilised ovum, or zygote. These are two landmarks between which 

 intervene two more extensive developments, on the one hand the sexual 

 generation, or prothallus, on the other the spore-bearing generation, or 



